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Polygons that are coplanar in your 3D space can be made to appear as if they are not coplanar by adding a z-bias to each one. This is a technique commonly used to ensure that shadows in a scene are displayed properly. For instance, a shadow on a wall will likely have the same depth value as the wall does. If you render the wall first and then the shadow, the shadow might not be visible, or depth artifacts might be visible. You can reverse the order in which you render the coplanar objects in hopes of reversing the effect, but depth artifacts are still likely.
An application can help ensure that coplanar polygons are rendered properly by adding a bias to the z-values that the system uses when rendering the sets of coplanar polygons. To add a z-bias to a set of polygons, call the IDirect3DDevice9::SetRenderState method just before rendering them, setting the State parameter to D3DRS_DEPTHBIAS, and the Value parameter to a value between 0-16 inclusive. A higher z-bias value increases the likelihood that the polygons you render will be visible when displayed with other coplanar polygons.
Offset = m * D3DRS_SLOPESCALEDEPTHBIAS + D3DRS_DEPTHBIAS
where m is the maximum depth slope of the triangle being rendered.
m = max(abs(delta z / delta x), abs(delta z / delta y))
The units for the D3DRS_DEPTHBIAS and D3DRS_SLOPESCALEDEPTHBIAS render states depend on whether z-buffering or w-buffering is enabled. The application must provide suitable values.
The bias is not applied to any line and point primitive. However, this bias needs to be applied to triangles drawn in wireframe mode.
// RenderStates D3DRS_SLOPESCALEDEPTHBIAS, // Defaults to zero D3DRS_DEPTHBIAS, // Defaults to zero
// Caps D3DPRASTERCAPS_DEPTHBIAS D3DPRASTERCAPS_SLOPESCALEDEPTHBIAS